1. Field of the Invention
The invention concerns a regulating device including a starting and stopping assisting arrangement for screw-type compressors which contain a compressor element, a minimum pressure valve which is mounted on the outlet of the compressor element and a vessel which is mounted between the compressor element and the minimum pressure valve, which regulating device contains means to control the motor.
2. Discussion of the Prior Art
The oil-injected screw-type compressors have been equipped until now with a regulating device provided with a starting and stopping aid which contains means to release the volume, after the compressor has come to a standstill and thus at zero rotational speed, which is situated between the outlet of the compressor element and the minimum pressure valve, in the atmosphere and which contains a relief valve which makes sure that the compressor element can further run idly, i.e. at a minimum pressure of about 0.5 to 1 bar which is sufficient to guarantee the oil injection, but whereby a minimum amount of air is drawn in.
With such a starting and stopping aiding arrangement, the succession of loaded and idle periods of the compressor provides a possible compressor regulation, and the means to control the motor are means which make the motor turn at an almost constant rotational speed, whereas the pressure in the vessel varies. Every loaded period, with maximum vessel pressure, is hereby followed by either an idle period with minimum pressure in the vessel sufficient to guarantee the oil injection in the compressor, or by a stop period with full blasting off of the vessel.
The blasting off of the volume between the outlet of the compressor element and the minimum pressure valve, until the pressure there becomes equal to the ambient pressure, is necessary with these known regulating devices to avoid very high current peaks when restarting. Indeed, given the output, the motor of the compressor is started with a starting current restriction, normally a star/D switch, to avoid current peaks. As a result, however, also the supplied motor torque drops, which becomes insufficient to start the compressor without blasting off the pressure at the outlet.
The blasting off results in a loss of energy, however, since compressed air is lost without being efficiently used. The blasting off produces a noise which must be possibly muffled, whereas the blast-off air must be blown off. Moreover, the means to blow off the air through the opening and closing of valves and air valves have quite a complicated construction.
The relief valve is necessary to restrict the number of starting and stop frequencies. With a star/D start, the number of starts and stops is limited to some fifteen per hour, as the motor would otherwise heat up too much.
Also this relief valve causes a loss of energy, since, also when the compressor, instead of stopping, turns on idly and does not produce any output, the motor still absorbs up to one fourth if its nominal output.